1. Field of the Invention
The present invention relates to an electrode which constitutes a main lens of an electron gun for a color picture tube.
2. Description of the Prior Art
FIG. 1 is a plan view of a color picture tube equipped with an electron gun of a conventional structure. A phosphor screen 3 on which phosphors of three colors are alternatingly applied is supported on the inner wall of a face plate 2 of a glass envelope 1. A triode which is an electron source is constituted by cathodes 6, 7, 8 and a first electrode G1 and a second electrode G2. Center axes 17, 18 and 19 of the cathodes 6, 7 and 8 are, respectively, in agreement with the center axes of apertures that correspond to cathodes of the first and second electrodes G1 and G2, and of the third and fourth electrodes G3 and G4, and of a shield cup 15, the third and fourth electrodes G3, G4 and the shield cup 15 constituting a main lens. The center axes 17, 18 and 19 are arranged on a common plane nearly in parallel with each other. The center axis 18 is also a center axis of the embodiment of electron gun. Cylindrical portions 117, 118, 119, 127, 128 and 129 are protruded from the opposing apertures of the third and fourth electrodes G3 and G4, the cylindrical portions being directed toward the interior of the electrodes. The cylindrical portions 117, 118, 119, 127, 128 and 129 play the role of shielding plates so that the electric field will uniformly infiltrate into the interior of the electrodes. Among these cylindrical portions 117, 118, 119, 127, 128 and 129 that work as shielding plates, the cylindrical portions 117, 119, 127 and 129 arranged on both sides have ends that are tilted at a predetermined angle with respect to the center axes 17 and 19.
Three electron beams emitted from the cathodes 6, 7 and 8 are incident upon the main lens along the center axes 17, 18 and 19. The center axes 17, 18 and 19 are referred to as initial paths of the electron beams. A focus voltage of about 5 to 9 KV is applied to the third electrode G3, and to the fourth electrode G4 is applied a voltage of a high as about 20 to 30 KV that is also commonly applied to the shield cup 15 and to a conductive coating 5 formed inside the glass envelope 1. Since the cylindrical portions 118 and 128 are symmetrically formed along the center axis 18, the main lens is symmetrical along the axis to focus the electron beam (central beam) that passes along the center axis 18 as a path. Accordingly, the center beam proceeds straight along the center axis 18. On the other hand, the main lens formed along the center axes 17 and 19 of the outer side has ends that are tilted at the cylindrical portions 117, 119, 127 and 129, and has the action to focus the electron beams as well as the action to deflect the electron beams as has been disclosed in Japanese Patent Laid-Open No. 63750/1982 (U.S. Ser. No. 307,572). Thus, the outer electron beams (electron beams that pass along the center axes 17, 19 as paths) incident upon the main lens along the center axes 17 and 19 of the outer sides, are converged by the main lens and receive the convergence power in the direction of center beam.
As described above, the three electron beams are focused on a shadow mask 4 and are converged so as to be superposed upon one another. The operation for converging the electron beams is called convergence and is particularly called static convergence (hereinafter abbreviated as STC) when the convergence takes place at the center of the screen. The electron beams are chromatically sorted by the shadow mask 4, and only those components that excite the phosphors of the corresponding colors are allowed to pass through apertures of the shadow mask to reach the phosphor screen. In order to scan the electron beam on the phosphor screen, furthermore, an external magnetic deflection yoke 16 is provided to surround the periphery of the glass envelope 1.
However, the apparatus of FIG. 1 has problems as described below. Namely, the outer beams incident upon the main lens along the initial paths 17 and 19 receive the convergence power in the third electrode G3 in the direction of the center axis 18. Therefore, the outer beams pass through portions that lie inside the center axes 17 and 19 of the main lens, whereby comma aberration is generated to form distorted spots on the screen.
FIG. 2 is a plan view of a color picture tube equipped with another electron gun of the conventional structure. What makes the structure of FIG. 2 different from that of FIG. 1 is that the main lens is constituted by a third electrode G32, a fourth electrode G42, a fifth electrode G52, a sixth electrode G62, and a shield cup 15. In FIG. 2, the fifth electrode G52 and the sixth electrode G62 have cylindrical portions 117, 118, 119, 127, 128 and 129 with tilted ends like the third and fourth electrodes G3, G4 of FIG. 1.
The three electron beams emitted from the cathodes 6, 7 and 8 are incident upon the main lens along the center axes 17, 18 and 19. In the example of FIG. 2, the main lens consists of a combination of two electron lenses, i.e., a so-called uni-potential focusing electron lens (UPF lens) constituted by the third electrode G32, fourth electrode G42 and fifth electrode G52, and a so-called bi-potential focusing electron lens (BPF lens) constituted by the fifth electrode G52 and sixth electrode G62. The main lens having the above-mentioned structure is called multi-step-focusing lens. The sixth electrode G62 assumes the same potential as the shield cup 15 and the conductive coating 5 formed inside the glass envelope 1, and is served with a high voltage of about 20 to 30 KV. The third electrode G32 and the fifth electrode G52 are served with a focus voltage of about 5 to 9 KV. There can be considered a case where a high potential common to the sixth electrode G62 is applied to the fourth electrode G42, and another case where the fourth electrode G42 is served with a low potential of about 400 to 1000 volts which is nearly equal to that of the second electrode G2.
The beam incident upon the main lens is focused by the above-mentioned two lenses (UPF lens and BPF lens). The main lens is symmetrically formed along the axis for the beam (central beam) that is incident along the center axis 18. Therefore, the center beam is focused by the main lens and proceeds straight through a trajectory along the center axis 18. On the other hand, of the electron lenses constituting the main lens formed along the center axes 17, 19 of the outer sides, the BPF lens formed by the fifth electrode G52 and the sixth electrode G62 has cylindrical portions that are tilted, and is capable of focusing the electron beam as well as deflecting it like the electron lens of FIG. 1, and makes it easy to establish the STC.
With the devices shown in FIGS. 1 and 2, the amount of deflecting the outer beams increases with the increase in the tilting angle at the ends of the cylindrical portions of the electrodes. To increase the tilting angle, a maximum value in the length of the cylindrical portions must be increased and a minimum value must be decreased. From the standpoint of manufacturing the electrodes, however, the upper limit of maximum value is about 50% of the inner diameter of the cylindrical portion and the lower limit of minimum value is about 2.5 times the thickness of the electrode. In fabricating the electrodes, therefore, if use is made of metal plates having nearly the same thickness, the maximum value decreases with the decrease in the diameter of the main lens. On the other hand, since the minimum value remains constant, the tilting angle decreases at the ends of the cylindrical portions. This decreases the amount of deflecting the outer beams, and it becomes difficult to establish the STC.
In order to solve such a problem according to the multi-step-focusing lens of FIG. 2, not only BPF lens but also the UPF lens, which consists of the third, fourth and fifth electrodes G32, G42, G52, is formed asymmetrically along the axis, in order to give deflection power to the electron beam and to compensate the lack of the amount of deflection.
FIG. 3 shows an example in which beam deflection means of an asymmetrical structure shown in U.S. Pat. No. 3,772,554 and U.S. Pat. No. 3,873,879 is applied to the UPF lens, in order to increase the amount of deflecting the beam. That is, between a pair of third and fourth electrodes G32 and G42, the center axis of the aperture of the fourth electrode G42 located on the side of the phosphor screen is slightly deviated outwardly relative to the center axis of the aperture of the third electrode G32, so that the outer beam is deflected toward the direction of center beam. Here, among the neighboring electrode apertures, the apertures on the side of the phosphor screen have diameters that are not smaller than the diameters of the apertures of the other side, so that the electrode apertures can all be secured with a cylindrical jig that is inserted from the side of the sixth electrode G62. The fourth electrode G42 is serviced with a high voltage Vo which is commonly applied to the sixth electrode G62, and the third and fifth electrodes G32 and G52 are served with a focus voltage Vf which is lower than the voltage Vo.
Even with the electron gun of the above-mentioned structure, however, the problem arises as described below like that of FIG. 1. That is, the outer beam 20 shown in FIG. 3 is deflected toward the center axis 18, and passes through a portion that is greatly deviated from the center axis of the aperture of the fourth electrode G42 of which the center axis is outwardly deviated. Therefore, the outer beam 20 receives different convergence power depending upon the portion on the side of the center axis 18 and the portion on the side of the center axis 17 to form a spot of a distorted shape on the phosphor screen.
FIG. 4 schematically illustrates the shapes of spots of the beams on the color picture tube. Spot shapes 31 and 32 of outer beams are shown on both sides of a spot 30 of the center beam. Hatched areas represent high brightness portions 301, 311, 321 that are called cores, and the peripheries thereof represent low brightness portions 302, 313, 322 that are called halos. In the spots 31 and 32, in particular, the halos spread in the lateral directions to deteriorate the vertical resolution of the color picture tube.
Such a problem also takes place even with the main lens of the structure in which a potential nearly equal to that of the second electrode G2 is applied to the fourth electrode G42. According to the method of FIG. 3, in this case, the center axis of aperture of the fifth electrode G52 on the side of the fourth electrode G42 must be deviated outwardly compared with the aperture of the outer side of the fourth electrode G42. Even with this structure, however, the outer beams pass through a portion inside the center axis of aperture of the fifth electrode G52 on the side of the fourth electrode G42, and the spot shapes on the screen are distorted like those shown in FIG. 4.
The method of deflecting the electron beam in two steps is employed in order to prevent the amount of deflecting the beam from varying relative to the focus voltage Vf when the potential of the fourth electrode G42 is lower than the potential of the third and fifth electrodes G32 and G52 as has been taught in Japanese Patent Laid-Open No. 53853/1980. In this case, the electron beam is often deflected by deviating the center axis of aperture even between the fifth electrode G52 and the sixth electrode G62. Even in this case, the spots of outer beams are distorted as shown in FIG. 4, as a matter of course.